I just used the "S-on" method on the development site and it worked. Some language difficulties, but you should be able to figure it out. You need a copy of the HTC / AT&T 2.2 ROM update, which appears to have been taken off the HTC website. I had a copy. The method has you copy a file to the phone and then run the ROM update - he says from the phone but means from the PC. I'd post a copy of the AT&T ROM, but I have no idea how.
Sent from my Liberty using XDA